The contributions of midbrain dopamine (DA) systems to drug reward and to drug sensitization are clear. It is not clear which brain circuits most importantly contribute to changes in DA neurons following drug exposure. The laterodorsal tegmental nucleus (LDTg) provides parallel cholinergic (ACh) and glutamatergic (GLU) inputs to ventral tegmental area (VTA) DA neurons that importantly control the activity of midbrain DA systems. In rats and mice ACh and GLU inputs to the VTA modulate the rewarding effects of cocaine and of opiates. Understanding the contributions of LDTg ACh or GLU inputs to the VTA to drug reward and addiction requires a means of selectively targeting these inputs. Preliminary results in Cre transgenic mice indicate that selective optogenetic excitation (via channelrhodopsin 2) of LDTg ACh but not GLU axons in the VTA is rewarding. Selective optogenetic inhibition of LDTg GLU cells (via halorhodopsin) reduces the induction of cocaine locomotor sensitization (an established model that quantifies behavioral adaptations induced by repeated drug exposure thought to reflect underlying neural changes important for drug addiction) but does not affect the acquisition of cocaine conditioned place preference (CPP - an established model that tests primary drug reward). This suggests that these LDTg GLU afferents to the VTA may be a site at which GLU plasticity is important for addictive behaviors to occur. Considering the above, this project will test the hypothesis that LDTg ACh afferents to the VTA are important for reinforcement and for drug reward, and that LDTg GLU afferents to the VTA are important for the induction of sensitization following repeated drug exposure. Aim 1 focuses on establishing that excitation of LDTg ACh but not GLU axons to the VTA is rewarding. Aim 2 focuses on determining the contributions of LDTg ACh afferents to the VTA to cocaine (typical stimulant drug) and to morphine (typical opiate drug) CPP, by optogenetically inhibiting LDTg ACh or GLU axons in the VTA during drug conditioning. Aim 3 focuses on establishing the importance of LDTg GLU inputs to the VTA in the induction of drug sensitization by inhibiting LDTg GLU axons in the VTA during repeated cocaine- or morphine-induced locomotion testing. The critical contribution of the proposed research will be to define, for the first time, the sourc of VTA glutamate and acetylcholine that are important for drug reward and for sensitization. Characterizing the role of select LDTg inputs to the VTA in drug reward and locomotor sensitization will inform future studies aimed at developing mechanistically based interventions, targeting a specific neural circuit, seeking to prevent the long-lasting adaptations in DA neuron function that underlie addictive behaviors. In line with the goals of the R15 funding mechanism these experiments will continue to expose several Loyola University Chicago (a primary educational institution) undergraduate students to research in Behavioral Neuroscience.